I've dubbed it "Super Game Boy" due to the color scheme and used that for the screen protector, cartridge label and battery label. "Super Game Boy" was actually a different thing – it was a cartridge for playing Game Boy games on a SNES, but at least the "Super" was taken from the original logo.
Some tidbits about soldering and two things you should definitely not do.
Oh, and really solder both, the front and the back before snapping off the pins. My plyers can't cut close to the surface, especially with the solder in the way, and i thought I'd be clever and solder the back and cut the front before soldering it. I tried that once and the pressure snapped the whole pin and solder pad right off of kite's board. I soldered it back and it somehow still works ... but jesus. Don't be me.
This is how I placed the back buttons and the cartridge.
I've decided to add normal buttons for L and R. The only reasonable place to put the shoulder buttons to is right above the battery compartment. There are two small spots where the buttons fit perfectly. If you place your buttons there, you can still hold your fingers rather naturally and have some space left for closing the battery cap and for a cartridge. This was the helper:
There won't be space left for a Cartridge reader. I've modded my cartridge to be used as a USB stick (which I will never use as an USB stick). It is possible to use the WIFI module solder points to work as an internal USB port but there's very little space. Barely enough for half a USB port, so you'd have to mod that quite a bit. I didn't connect the USB port so it's only used to keep the cartridge in place.
A little more in depth info and some internals.
This is how the controllers look from the inside. The shoulder buttons each have their own board and little rubber cap. I've used the spare Game Boy buttons and the remaining two button wells from the SNES controller – so they're more or less the same as the X and Y buttons on the front.
There are some who use the SNES shoulder buttons for their GBZ. The curvature sees to fit, but cutting the case would require a lot of work and I wasn't so sure if I could pull it off. Fleder had those: viewtopic.php?f=43&t=14
Anyway, I also decided not to use tactile buttons because I don't like how "clicky" they are. I've used them for a modded PS4 controller and in the end never used them because they annoyed me so much. The rubber pads feel very natural and in line with the buttons on the front.
The only thing I would do differently: I've used the rubber caps from the shoulder buttons that I've shown above, but the normal rubber caps would probably feel slightly better due to the wider tip.
Above you're seeing the back of the Game Boy, with the shoulder button boards and the mangled USB port in place. The Front is used up completely by Kite's SAIO. The button boards and buttons perfectly fit into the space above the battery compartment and the hand position is still quite natural.
And as mentioned above, the USB port does nothing but keeping the cartridge in place, but at least there's enough space for a full cartridge : )
The battery is removable and I love it
I've applied some black ink with a brush to the side of the battery. When trying to push the battery in, the ink will leave stains where still too much material is. Grind those bits. I've seen my dentist do that with tooth fillings and it seemed like a good approach.
Adding pogo pins (actual Samsung S2 pins bought from eBay) was pretty straight forward: cut a hole, shove the pins through and glue them. If you put the battery in, you'll can see the battery's pin placement from the inside, so drill a little hole and check how it's aligned. Do check which pins are connected: It's the first and the third pin on the battery. Hollyhoo used different pogo pins and had to connect pin 1 and 2 of his pogo pins due to the pin distance. I followed that blindly and had to rework that, haha.
The battery is also not "wide" enough for the compartment. Hollyhoo used a 3d printer to make a little attachment, but I don't have one. I noticed that the width of the plastic pieces from the screen cutout was the perfect size, so that's what keeping my battery in place (upper right and lower left on the image above). I really like this part.
I also tried to make the battery look like a real battery as well as I could. It includes a list of Kite's button commands, disguised as battery label gibberish.
I suppose the best way would be to get someone who has equipment that can print and cut. I'll replace this as soon as I have the chance. If someone still has one of those silkscreen "dot matrix with stereo sound" glasses or a screen surround from Dom...